1. Field of the Invention:
This invention relates to apparata and methods for manufacturing finished flat glass products. More particularly, this invention involves automated and energy efficient improvements in the design of electric furnaces and refiners, glass sheet formation, glass cutting devices, and in glass transfer and tempering mechanisms.
2. Description of the Prior Art:
Various techniques are presently being used to manufacture flat sheet glass. Typically, premixed glass-forming materials are fed onto the surface of a bath of molten glass contained in a furnace. In the fuel firing of the regenerative tank type furnaces, the materials are melted by hot gases from flames playing across the furnace above the glass surface. In the more modern electric furnaces, heat is produced by passing electric current through the bath of molten glass between electrodes immersed in the glass. Also, a combination of both heating methods are sometimes employed.
The furnaces described above assume various shapes. Early regenerative, fuel fired, tank type furnaces were generally horizontal and rectangular in shape with raw material received in one end and molten glass formed in a continuous sheet on the opposite end. This furnace at one time enjoyed considerable popularity in view of the abundance of relatively cheap natural gas energy resources. However, as natural gas fuel became scarce and therefore expensive, the energy consumption deficiencies of the regenerative furnace soon became apparent. In particular, the horizontal regenerative furnace experienced considerable heat loss because of its relatively large exposed cross-sectional areas. Therefore the trend in recent years has been to employ vertical electric furnaces. These furnaces are characterized by smaller cross-sectional area, and therefore less heat loss. However these furnaces likewise have not been without problems. A perennial problem with electric furnaces has been heat localization around the electrodes, and the integrity of the furnace walls surrounding the localized electrode heat pockets. Furtheremore, normal electrode wear requires regular replacement, which has resulted in shut-down of the furnace.
In conventional glass manufacturing furnaces, the glass batch after being melted in the furnace is refined in adjacent refining and melting sections. The adjacent refining section, normally equipped with auxiliary heating means, provides a zone for temperature equalization whereby air bubbles are eliminated and glass homogeneity is effected. Typical glass melting furnaces and refiners are found in U.S. Pat. Nos. 3,636,227; 3,936,290; 3,997,316; 3,998,619; 4,011,070; 4,012,218. Typical glass furnace electrode assemblies are found in U.S. Pat. Nos. 2,798,892; 2,978,526; 3,409,725; 3,517,107; 3,576,385; 3,681,506; 3,740,445; and 3,813,468.
After reaching a state of equilibrium within the refiner stage, the molten glass is withdrawn from the refiner. In the Pittsburgh or Pennvernon sheet glass drawing apparatus a series of pairs of rolls provide tractive force which draws glass upwardly from a bath of molten glass. The thickness of the continuous ribbon of glass is maintained by the speed of the rolls drawing the semi-molten glass from the reservoir. As shown in U.S. Pat. No. 3,420,650 molten glass flows out of the furnace through an aperture in the wall of the furnace and into the bite of a pair of forming rolls which form the molten glass into a continuous ribbon. In the float process, the glass sheet passes from the furnace to forming rolls which determine thickness, and then flows onto a molten tin bath which imparts an ideal flatness to the glass ribbon. In the float process, glass thickness is further controlled by manipulating the glass temperature and speed of advancement to longitudinally stretch the glass, or by installing longitudinally extending fenders within the molten metal bath structure to limit the lateral spreading of the glass as it advances across the molten bath. While these techniques have proven satisfactory in the past, the reproducibility of a precise glass ribbon thickness has been difficult, primarily because of the rather imprecise metering of the molten glass from the furnace.
In conventional glass manufacturing plants, when the glass ribbon is removed from the molten metal bath, it then enters a covered annealing lehr where the temperature of the glass is lowered gradually from the semi-molten state to a rigid and near room temperature cooled glass. The length of the annealing lehrs used in the prior art varies, with some lehrs being several hundred feet in length. The annealing process results in a hardened manageable glass product that can be cut with a diamond or scoring wheel. However, at this stage in the glass manufacturing process, the annealed glass is lacking of impact strength and is susceptible to breakage. Also, when breakage occurs, the annealed glass usually breaks into hazardous jagged fragments.
After annealing, which to a certain extend occurs naturally as the glass is removed from the molten metal bath, it may be desirable to submit the glass ribbon to a tempering process which results in a safety glass. To temper glass, it is necessary to reheat the glass from the annealed state and to return the glass to semi-molten temperatures. Thereafter, sudden chilling of the glass sheet on both sides, simultaneously, with cold air or oil is required. This tempering phase improves the impact strength of the glass sheet by a factor of 4 over the annealed glass sheet. Moreover, when tempered glass is broken, it breaks into relatively harmless, "marble" shaped pieces, rather than the dangerous sharp pointed fragments of broken annealed glass. Unfortunately, the glass sheet once tempered cannot be recut with the ease of the annealed product, and therefore tempered glass is to a larger extent subject to breakage when being cut. It has therefore become necessary that the desired size of the sheet be exact before the tempering phase is performed. Typical methods and apparata for tempering glass sheets are disclosed in U.S. Pat. Nos. 3,488,173; 3,647,409; 3,734,706; 3,841,855; 3,875,766; 3,881,906; 3,923,488; 3,994,711; 3,929,442; and 4,004,901.
In conventional glass cutting apparata, the glass sheet is typically cut by scoring the sheet and then severing the sheet along the score line. Generally, glass sheets of different size are cut by multiple passes of the scorer/severing device. In order to increase the glass production rate, cutter mechanisms of the prior art were reciprocated across the glass ribbon at increased velocity. However, because of the proportionally irregular quality of the high velocity cuts, other techniques were developed for increasing production. As disclosed in U.S. Pat. No. 3,703,115 the cutting means is transported in the same direction as the moving glass ribbon and at substantially the same velocity. As a result, the reciprocating velocity of the cutting means can be discriminately selected. Nevertheless, however, this improvement in cutting speed is limited by the existence of only a single cutting mechanism, and the overall complexity of the system is increased as a result of these velocity control mechanisms.
U.S. Pat. No. 3,983,771 discloses an apparatus for precise subdivision of glass sheets. The subdivision is accomplished by combining the functions of wheel holders and spacing means in a row of spacer blocks whose contiguous surfaces are ground flat and polished with a high degree of accuracy, thereby establishing the spacing between scoring wheels to a high degree of accuracy. Unfortunately, with this cutting scheme, spacer blocks must be manually moved or replaced in order to change the spacing between adjacent scores. Predictably, this feature results in decreased flexibility of the glass cutting operation. Other glass cutting inventions which are considered to be of interest are disclosed in U.S. Pat. Nos. 3,165,017; 3,424,357; 3,754,884; 3,934,995; 4,004,900; and 4,010,677.